1. Field of the Invention
This invention relates to a structure for a movable platen for supporting a portion of an injection mold in an injection molding machine. More particularly, the present invention relates to a movable mold supporting platen that has improved rigidity that is slidably carried on an injection molding machine base.
2. Summary of the Prior Art
In general, machines for injection molding of plastics articles include a pair of fixed platens that are spaced from each other and that are interconnected by generally four parallel tie bars that have their axes positioned to define a rectangular array. One of the fixed platens remains stationary and is adapted to support one portion of a two or multiple piece injection mold that when assembled or engaged defines a mold cavity to correspond with the outline of a desired molded part. A movable platen is slidably carried on the tie bars between the fixed platens and is adapted to carry a cooperating portion of the mold so that when the movable platen is moved toward the mold-portion-carrying fixed platen the two mold portions come into contact to define therebetween a mold cavity for forming the desired part.
The movable platen is generally a plate-like structure that is of rectangular configuration and includes four bores at the respective corners, through each of which a tie bar extends. A movable platen actuation system is positioned between the non-mold-carrying fixed platen and the movable platen to cause the movable platen to move along the tie bars toward or away from the mold platen, and also to hold the movable platen firmly in position when the mold portions are together, to prevent separation of the molds as molten material is injected into the mold cavity under high pressure.
Injection molding machine platens are typically block shaped as shown in U.S. Pat. No. 5,188,850 to Hirata et al.; U.S. Pat. No. 5,066,217 to Fukuzawa et al.; U.S. Pat. No. 5,110,283 to Bluml et al.; U.S. Pat. No. 5,123,834 to Joyner; U.S. Pat. No. 5,162,782 to Yoshioka; U.S. Pat. No. 5,192,557 to Hirata et al; U.S. Pat. No. 5,593,711 to Glaesener; and U.S. Pat. No. 5,776,402 to Glaesener. In each of these patents, the mold platen is substantially block shaped having a substantially rectangular side which supports a mold half. During clamp-up of mold halves, a force is generated against the mold mounting face of the platen. As is typical with such block shaped platens, the mold mounting face is caused to concavely deform, forcing the top and bottom edges of the side towards the oncoming force and causing the platen to bend and provide tension across the backside of the mold platen. As a result, under the clamp-up force the center of the platens separate causing a gap between the mold halves, and in some cases, the formation of flash is a by-product.
U.S. Pat. No. 4,615,857 to Baird discloses an encapsulation means and method for reducing flash during mold operations. In accordance with this device, it is alleged that injection and transfer molding of plastic is performed in a manner that virtually eliminates flash. Deflection of the mold press therein is measured with the mold press in the clamping configuration. The support structure of the mold is arranged to apply an equal force to the mold face by configuring supporting pillars and bars such that they act as individual springs against the mold face and in response to the force generated while the mold is in the clamping configuration. The spring constants and the lengths of the supporting pillars and bars are calculated to account for the actual deflection found in the mold press and thereby producing a uniform pressure on the molds during clamping of the press.
Accordingly, the flexure of the mold platens during mold sealing is measured and the compressibility of the mold mounting blocks of the device are adjusted as a function of lateral position on one or both platens in order to compensate for platen flexure, so that a constant force is applied to the parting surface of the mold, independent of lateral position on the parting surface. The stiffness and length of the mounting blocks define the force provided by the mounting blocks and are determined in accordance with predetermined formulas depending upon the position of the block on the platen and the predetermined platen deflection. While Baird compensates for deflection, the method and apparatus by which this is achieved is quite complex requiring individual designs for particular forces to be generated with particular molds. Accordingly, a uniformly applicable design is not achieved.
FIGS. 5a and 5b disclose a prior art molding platen having a shape slightly different than the block shape platens discussed above in the cited patents. As shown in FIG. 5a, the mold platen has a profile including several openings therethrough, a front wall and a back wall. As shown in FIG. 5b, a plurality of slots and ribs extend toward the back wall which can and has a smaller surface area than the front wall of the platen. The platen also includes bores at each corner thereof for receiving tie-bars that resist the force FC between the platens during mold clamp-up. Each tie bar carries a resistance force FR as shown. The plurality of ribs and slots are provided for decreasing the weight of the platen. That is, the front mold mounting face is under compression during mold clamp-up while the back wall is under tension as in a simple beam. The tie bars are drawn inwardly and deform to conform with platen face movement, as shown by the dotted line and arrows in FIG. 5a, thereby causing the mold face to bend and have a concave configuration during molding, similar to the platens of the aforementioned patents. Accordingly, despite the design of the mold platen of FIGS. 5a and 5b, mold face bending is not compensated for and the possibility of the creation of flash still exists. Since both sides of the platen bend, the supports at the corners of the tie bars also bend resulting in uneven loading of the tie bar supports. This causes bending of the tie bars and high stress concentrations leading to premature fatigue failure. Arrows C show how the tie bars are bent during clamp-up.
There exists, therefore, a need for an injection molding machine having a simply designed and lightweight platen which includes means for eliminating or reducing platen deflection during mold or press clamp-up and which substantially eliminates the creation of flash, during injection molding.